Generally, a solid polymer electrolyte fuel cell employs a solid polymer electrolyte membrane. The solid polymer electrolyte membrane is a polymer ion exchange membrane. The solid polymer electrolyte membrane is interposed between an anode and a cathode each including a catalyst layer (electrode catalyst) and a gas diffusion layer (porous carbon) to form a membrane electrode assembly (MEA). The membrane electrode assembly is interposed between separators (bipolar plates) to form the fuel cell. Normally, in use, predetermined numbers of the fuel cells and the separators are stacked together to form a fuel cell stack.
In the membrane electrode assembly, a solid polymer electrolyte membrane in the form of a thin film is used. Therefore, it is required to prevent damage of the solid polymer electrolyte membrane caused by differential pressure of reactant gases or mechanical stress applied to the solid polymer electrolyte membrane.
In this regard, for example, a sealing structure of a solid polymer electrolyte fuel cell disclosed in Japanese Laid-Open Patent Publication No. 05-021077 is known. As shown in FIG. 4, a solid polymer electrolyte membrane 1 is interposed between an anode 2 and a cathode 3. A separator 4 having a fuel gas channel 4a is disposed on the anode 2, and a separator 5 having an oxygen-containing gas channel 5a is disposed on the cathode 3.
Protective membranes 6 comprised of frame-shaped fluorine-resin sheets are inserted on both surfaces at the outer circumferential ends of the solid polymer electrolyte membrane 1. When the solid polymer electrolyte membrane 1, the anode 2 and the cathode 3 are combined together by thermal compression bonding process, the protective membranes 6 are formed integrally therewith. Gas seal members 7 are inserted between the separators 4, 5 around the anode 2 and the cathode 3 like frames of the anode 2 and the cathode 3.